Since a tire has a geometry that exhibits symmetry of revolution about an axis of rotation, the geometry of the tire can be described in a meridian plane containing the axis of rotation of the tire. For a given meridian plane, the radial, axial and circumferential directions respectively denote the directions perpendicular to the axis of rotation of the tire, parallel to the axis of rotation of the tire, and perpendicular to the meridian plane. In what follows, the expressions “radially on the inside” and “radially on the outside” respectively mean “closer to the axis of rotation of the tire in the radial direction” and “further away from the axis of rotation of the tire, in the radial direction”. The expressions “axially on the inside” and “axially on the outside” respectively mean “closer to the equatorial plane, in the axial direction” and “further away from the equatorial plane, in the axial direction”, the equatorial plane being the plane perpendicular to the axis of rotation of the tire and passing through the middle of the tread surface of the tire.
An airplane tire is characterized by a nominal pressure in excess of 9 bar and a nominal level of deflection greater than or equal to 32%. The nominal pressure is the nominal inflation pressure of the tire as defined, for example, by the standards laid down by the Tire and Rim Association or TRA. The nominal level of deflection of a tire is, by definition, its radial deformation, or variation in radial height, when it changes from an unladen inflated state to a statically loaded inflated state under nominal load and pressure conditions as defined, for example, by the TRA standard. It is expressed in the form of a relative deflection, defined by the ratio of this variation in radial height of the tire to half the difference between the outside diameter of the tire and the maximum diameter of the rim measured on the rim flange. The outside diameter of the tire is measured under static conditions in an unladen state inflated to the nominal pressure.
A tire in general comprises a crown comprising a tread intended to come into contact with the ground via a tread surface, two beads intended to come into contact with a rim and two sidewalls connecting the crown to the beads. A radial tire, such as is generally used on an airplane, more specifically comprises a radial carcass reinforcement and a crown reinforcement both as described, for example, in document EP1381525.
The radial carcass reinforcement is the tire reinforcing structure that connects the two beads of the tire. The radial carcass reinforcement of an airplane tire generally comprises at least one layer of carcass reinforcement, each layer of carcass reinforcement being made up of mutually parallel reinforcing elements, usually textile, making an angle of between 80° and 100° with the circumferential direction.
The crown reinforcement is the tire reinforcing structure radially on the inside of the tread and at least partially radially on the outside of the radial carcass reinforcement. The crown reinforcement of an airplane tire generally comprises at least one layer of crown reinforcement, each layer of crown reinforcement being made up of mutually parallel reinforcing elements coated in a polymer coating material. Within the layers of crown reinforcement, a distinction is made between the layers of working reinforcement that make up the working reinforcement, usually made up of textile reinforcing elements, and the layers of protective reinforcement that make up the protective reinforcement and are made of metallic or textile reinforcing elements positioned radially on the outside of the working reinforcement.
During the manufacture of an airplane tire, a layer of working reinforcement is usually created by a zigzag winding or winding in turns of strips made up of textile reinforcing elements; around a cylindrical manufacturing device by performing an axial translational movement of the strip for each turn of winding so as to obtain the expected axial width of layer of working reinforcement. The layer of working reinforcement is thus made up of axially juxtaposed strips. What is meant by zigzag winding is a winding in a curve formed of undulations that are periodic, either over half a period per turn of winding or over one period per turn of winding, the angle of the textile reinforcing elements of the strips generally being comprised between 8° and 30° with respect to the circumferential direction. For a layer of working reinforcement that is created by winding in turns, the angle of the textile reinforcing elements of the strips is generally comprised between 0° and 8° with respect to the circumferential direction. Whatever the type of winding of the strips, the angle of the textile reinforcing elements of the strips is generally less than 30° with respect to the circumferential direction. For this reason, the strips and the resulting working layer are said to be substantially circumferential, which means substantially circumferential in direction with undulations of limited amplitude about the circumferential direction.
The reinforcing elements of the layers of working reinforcement are mutually parallel, which means to say that the distance between the geometric curves of two adjacent reinforcing elements is constant, it being possible for the geometric curves to exhibit periodic undulations.
The reinforcing elements of the layers of carcass reinforcement and of the layers of working reinforcement, for airplane tires, are usually cords made up of spun yarns of textile filaments, preferably made of aliphatic polyamides or aromatic polyamides. The reinforcing elements of the layers of protective reinforcement may be either cords made up of metallic threads, or cords made up of spun yarns of textile filaments.
The mechanical properties under tension (modulus, elongation and force on rupture) of the textile reinforcing elements are measured after prior conditioning. What is meant by “prior conditioning” is that the textile reinforcing elements are stored for at least 24 hours, prior to measuring, in a standard atmosphere in accordance with European standard DIN EN 20139 (a temperature of 20±2° C.; a relative humidity of 65±2%). The measurements are taken in a known way using a tensile test machine made by ZWICK GmbH & Co (Germany) of type 1435 or type 1445. The textile reinforcing elements are subjected to tension on an initial length of 400 mm at a nominal rate of 200 mm/min. All the results are averaged over 10 measurements.
A polymer material, such as the polymer coating material used for the textile reinforcing elements of the layers of working reinforcement is mechanically characterized, after curing, by tensile stress—strain properties that are determined by tensile testing. This tensile testing is performed, on a test specimen, using a method known to those skilled in the art, for example in accordance with International Standard ISO 37, and under normal temperature (23+ or −2° C.) and humidity (50+ or −5% relative humidity) conditions defined by International Standard ISO 471. For a polymer mixture, the tensile stress measured for a 10% elongation of the test specimen is known as the elasticity modulus or tension modulus at 10% elongation and is expressed in megapascals (MPa).
In use, the mechanical stresses of running, resulting from the combined action of the nominal pressure, of the load applied to the tire which may vary between 0 and 2 times the nominal load, and of the speed of the airplane, introduce tension cycles into the reinforcing elements of the layers of working reinforcement.
These tension cycles generate, within the polymer coating material of the reinforcing elements of the layers of working reinforcement, sources of heat, particularly at the axial ends of the layers of working reinforcement. These sources of heat are localized hot spots where the removal of heat is difficult, because the heat has to be able to spread either through the polymer coating material or through the textile reinforcing elements. However, the polymer coating material, because of its low thermal conductivity, is a poor conductor of heat. Likewise, the textile reinforcing elements, because of their low thermal conductivity, cannot make an effective contribution towards the removal of heat. This results in excessive heating of the polymer coating material, which is prejudicial to its correct mechanical integrity and is likely to cause it to degrade, thus leading to premature tire failure.
Various technical solutions have been conceived of in an attempt to create a path for the removal of the heat generated in the working reinforcement. Documents EP1031441 and JP2007131282 disclose thermally conducting polymer materials with improved thermal conductivity. Document EP1548057 proposes polymer materials that include carbon nanotubes to increase the thermal conductivity. Document EP1483122 describes a thermal drain, in the form of metallic cables laid in a meridian plane and inserted at the end of the working reinforcement. Finally, document KR812810 proposes a thermally conducting insert, which may be metallic and is arranged at the end of the working reinforcement.